This invention concerns exchanges of the kind having a distribution frame and subscriber interface cards. There is a growing need to provide an analogue switch facility between the distribution frame and the interface cards within the exchange or in a multiplexor of the exchange. Such an analogue switching facility is sometimes called an Electronic Cross Connect Frame (ECCF). An ECCF permits "sparing" (the continued provision of service to a subscriber whose interface card has failed by switching the subscriber to a spare interface card) and also alternative service provision e.g. when a subscriber requests a service provided only by a different physical interface circuit which can only be provided immediately by the network operator switching in a different interface card.
Ideally, a network operator would be able immediately to switch any subscriber to any of the different types of interface cards provided within the exchange or its multiplexor. However, such an ECCF would not be cost effective with present or forseeable technology.
For example, for an exchange (or multiplexor) with 900 subscriber lines for which the connections may be of the 2-wire or 4-wire type, then a three stage switching matrix of the kind shown in FIG. 1 of the accompanying drawings would be necessary to provide such complete interconnectivity. Each indicated line may represent a 4-wire connection. The matrix would then be constituted by 144566 four pole switches. Using relays, with their associated circuit boards, edge connectors, drivers etc., the cost would be in excess of $230 per line and the volume would be considerable.
As an alternative approach, if separate non-interconnecting but fully available switches are implemented for modules each of 60 subscriber lines, the number of switches required would be 33880 four pole switches. Again, the cost for a 900 line exchange would be prohibitive.
If the switching matrix was implemented with solid-state cross points, the volume (but not to any great extent, the cost) would be reduced. For example, either gated diodes or high voltage FETs (Field Effect Transistors) may be used instead of relays. Gated Diodes, for three stage switching, would have a path resistance of approximately 100 ohms. High voltage FETs are prohibitively expensive. It is difficult also to provide well matched impedances using such devices (a balance about earth of some 70 ohms).
It is an object of the present invention to provide an electronic cross connect frame, for use in an exchange or a multiplexor of an exchange, wherein the foregoing disadvantages are minimised.